Increased levels of storage capacity in floppy and hard disk drives are a direct result of the higher track densities possible with voice-coil and other types of servo positioners. Previously, low track density disk drives were able to achieve satisfactory head positioning with leadscrew and stepper motor mechanisms. However, when track densities are so great that the mechanical error of a leadscrew-stepper motor combination is significant compared to track-to-track spacing, an embedded servo-pattern is needed so that the position of the head can be determined from the signals it reads.
Conventional hard disk manufacturing techniques often include writing servo-patterns on the media of a head disk assembly (HDA) with a specialized servowriter instrument. Laser positioning feedback is used in such instruments to read the actual physical position of a recording head used to write the servo-patterns. Unfortunately, it is becoming more and more difficult for such servowriters to invade the internal environment of an HDA for servowriting because the HDAs themselves are exceedingly small and depend on their in-place covers and castings for proper operation. Some HDAs are the size and thickness of a plastic credit card. At such levels of microminiaturization, traditional servowriting methods are inadequate.
Conventional signals of servo-patterns typically comprise short bursts of a constant frequency signal, very precisely located offset from a data track's center line, on either side. The bursts are generally, but not required to be, located in a trajectory within a track. The bursts are written in a sector header area, and can be used to find the center line of a track. Staying on center is required during both reading and writing. Since there can be sixty, or even more, sectors per track, that same number of servo-pattern areas must be dispersed around a data track. These servo-pattern areas allow a head to follow a track center line around a disk, even when the track is out of round, as can occur with spindle wobble, disk slip and/or thermal expansion. As technology advances provide smaller disk drives, and increased track densities, the placement of servo-patterns must also be proportionately more accurate.
Servo-patterns are conventionally written by dedicated, external servowriting equipment, and typically involve the use of large granite blocks to support the disk drive and quiet outside vibration effects. An auxiliary clock head is inserted onto the surface of the recording disk and is used to write a reference timing pattern. An external head/arm positioner with a very accurate lead screw and a laser displacement measurement device for positional feedback is used to precisely determine transducer location and is the basis for burst placement and spacing of bursts in successive tracks. The servowriter requires a clean room environment, as the disk and heads will be exposed to the environment to allow the access of the external head and actuator.
U.S. Pat. No. 4,414,589 to Oliver et al. describes servowriting wherein optimum track spacing is determined by positioning one of the moving read/write heads at a first limit stop in the range of travel of the positioning means. A first reference burst is then written with the moving head. A predetermined reduction number or percentage of amplitude reduction X%, is then chosen that is empirically related to the desired average track density. The first reference burst is then read with the moving head. The moving head is then displaced away from the first limit stop until the amplitude of the first reference burst is reduced to X% of its original amplitude. A second reference burst is then written with the moving head and the moving head is then displaced again in the same direction until the amplitude of the second reference burst is reduced to X% of its original value. The process is continued, writing successive reference bursts located in successive tracks and displacing the moving head by an amount sufficient to reduce the amplitude to X% of its original value, until the disk is filled with reference bursts in tracks (i.e., a propagation pattern). The number of reference bursts so written is counted and the process is stopped when a second limit stop in the range of travel of the positioning means is encountered. Knowing the number of tracks written and the length of travel of the moving head, the average track density is checked to insure that it is within a predetermined range of the desired average track density. If the average track density is high, the disk is erased, the X% value is lowered and the process is repeated. If the average track density is low, the disk is erased, the X% value is increased and the process is repeated. If the average track density is within the predetermined range of the desired average track density, the desired reduction rate X%, for a given average track density, has been determined and the servowriter may then proceed to the servowriting steps, using the collection of reference bursts written as a propagation pattern. This technique cannot accommodate changes in reference levels which may be required across the disk surface.
The process of servowriting using only the internal recording transducer and product actuator, (one form of self-servowriting) is thus generally known to involve a somewhat rigid application of three largely distinct subprocesses: writing and reading magnetic bursts to provide precise timing; positioning the recording transducer at a sequence of radial locations using the variation in readback signal amplitude from propagation bursts as a sensitive position transducer; and writing the actual product servo-pattern at the times and radial locations defined by the first two subprocesses. Again, such techniques currently suffer from exposure to changing conditions across the disk surface, and, in addition, to manufacturing tolerances in the HDAs themselves.
What is required are systems and methods for self-servowriting which are more flexible and which overcome the deficiencies of the presently known self-servowriting techniques.